Uncomfortable was the beauty queen's ride atop the float. What is the subject?

The simple subject is "ride".

The complete subject is the beauty queen’s ride atop the float. Just because the word "uncomfortable" begins the sentences, it doesn't readily make it the subject.

This is called inversion. It simply means putting a verb before its subject. However, it doesn’t affect the innate function of each word. There are around 18 types of subject-verb inversion. The sentence above is an example of intro adjective inversion since the beginning word is an adjective that describes the subject.